I would not go so far as to call it drm, but I get the idea behind your argument.
Besides, what prvented GOG from being open up front like „we are creating a new webpage and our internal tests (ok, this would be a lie, because GOG never tests anything) have shown that IE11 is out and we decided because of reasons a, b and c to not change that.“ From my point of view that would be much better way to be open as they always promised to be , but are not anymore.
Do not buy anything until they deliver what you, the customers want. Vote with your wallet.
DRM is how I see it. But I get that people might not see it as that, but at least your getting my point.
Nothing stopped them. GOG say yeah we are doing the whole F DRM thing then you need certain browsers to use their site. You may well just use Galaxy or Steam tbh.
As I said IE11 might well be used by little, but every other store supports it, MS still support it, its just GOG being their usual anti-customer self in my eyes. People have been saying that Epic might cause the end of GOG, its these sort of anti-customer decisions (the Epic store works under IE11, and they don't have such a thing for auto playing videos either), that will cause GOG to go under.